In a time of increasing geopolitical uncertainty and attacks on major military alliances like NATO, peacekeeping alliances between individual nations are more important than ever before. 

The war-deterring benefits of these alliances extend beyond the nations directly involved, according to new research. Indirect relationships between nations with common allies are just as effective as direct relationships between allies, suggesting that the concept of “friends of friends” applies not just to people but also nations.

The study, conducted by political scientists in the U.S. and China and published Thursday in the journal Science Advances, found that nations with up to three degrees of separation from a direct ally are unlikely to engage in conflict. 

Having common allies seems to be a more powerful peacekeeping tool than we’ve realized. While it’s known that countries with military alliances are less likely to go to war, the new study is the first to suggest that countries friendly to those within an alliance are also less likely to get wrapped up in conflicts.

As the study’s authors noted, these cross-national alliances are modeled in a similar way to human social networks.  

“Alliances matter more than one might expect,” Dr. Skyler Cranmer, the study’s lead author and a political scientist at The Ohio State University, told The Huffington Post. “It’s not just alliance relationships that matter, but indirect relationships as well. … Much like people, countries tend to [have] good relationships with the friends of their friends (and even the friends of their friends of their friends).” 

For the study, Cranmer and her colleagues analyzed all major military conflicts between the years of 1965 and 2000. They included all conflicts in which one country used military force against another. Because most nations don’t have the ability to engage in conflicts from afar, the analysis focused on conflicts between neighboring states. 

They found that a peacekeeping alliance between two nations can extend for up to three degrees of separation before it weakens. That means that a country is not only less likely to go to war with its allies, but also with its allies’ allies, and it’s allies’ allies’ allies (is your tongue tied yet?). 

“What surprised us was that the suppressive effect on conflict did not decrease until four degrees of separation,” Cranmer said. “We had expected it to be kind of like a game of telephone, where suppressive effect on conflict would get weaker and weaker as degrees of separation increased.”

Instead, she explained, “that suppressive effect stayed more-or-less stable out to three degrees, and then collapsed completely at four degrees of separation.” 

For countries within three degrees of separation, the likelihood of engaging in a conflict was around 3 percent. However, when the separation jumped up to four degrees, the countries’ likelihood of engaging in conflict doubled. 

Then, the researchers divided the world up into “communities,” defined as groups of nations that are more connected to one another through their alliances than they are to other nations. They found that communities were almost entirely made up of nations within three degrees of connection ― which explained why there was so little conflict between them.

However, countries that had four or more degrees of separation were almost always in different communities, and were therefore more likely to engage in conflict. 

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